LeCroy LC564DL, Service Manual

Page: 30 / 210

4-6 Theory of Operation
Flash PROM
Two Intel 28F008-compatible 1MB PROMs (A24 and A25), are used.
From a hardware point of view, a flash PROM is the same as an EPROM in read
mode. To write to it, however, a programming voltage (Vpp) needs to be applied to
a pin. The 12V voltage is generated by a switching regulator (A26), controlled by a
logic level (_EPPP).
NVRAM
This chip is powered through the lithium battery (VCT) when power is off. The chip
select is held high through a pull-up resistor to VCT to avoid accidental overwriting
while power is off.
Interrupt Controller
In order to keep compatibility with both 68k hardware and software, it is necessary
to use a chip that prioritizes several interrupt sources. This is done by a NEC chip,
an uPD71059. It scans eight interrupt pins and sends a unique interrupt to the
processor when a (non-masked) interrupt appears.
Interrupt levels are assigned the following:
level 0 (lowest priority) acquisition board
level 1 small peripherals, unused
level 2 RS232
level 3 GPIB
level 4 small peripherals and acquisition board, unused
level 5 real time clock
level 6 time base (acquisition board)
level 7 (highest priority) only for test purposes. Linked to level 2 for
debugging.
Floppy Controller
The floppy controller chip directly interfaces to a double or high-density disk drive.
The floppy controller has a digital 8-bit input/output register, which is used to read
several status lines from the drive (disk inserted, etc.).
In principle, the controller is able to provide an interrupt when accesses are
completed. As these accesses are performed in non-time critical paths of the
program, The interrupt line has been wired to the input register, so that the program
has to poll this register until the interrupt line goes active.
The controller has it’s own timebase, a 24MHz crystal.
Centronics
Both external and internal Centronics ports are write-only. The data get latched in a
74HCT374 register (A45 for internal Centronics, A55 for external).